1. Field of the Invention
Embodiments of the present invention relate, in general, to amphibious aircraft and more particularly to the interaction between a sponson and wingtip of an amphibious aircraft.
2. Relevant Background
An amphibious aircraft is an aircraft that can operate on both land and water. While on land landing gear extending from the fuselage of the aircraft or similar structure allow the aircraft to operate as a conventional land-based aircraft. When operating in the water amphibious aircraft generally fall into two categories: 1) float-planes and 2) hull-type planes. In float-planes, a conventional aircraft is mounted on two external floats, otherwise known as pontoons. When operating in water only the pontoons or external floats reside in the water. In a hull-type aircraft the actual aircraft fuselage is designed to rest in the water without the need of floats and acts much like a boat with respect to water functionality.
Each type of amphibious aircraft has disadvantages. One disadvantage of float-plane type amphibians is the large amount of increased aerodynamic and hydrodynamic drag. Another is the substantial weight caused by the floats and mounting structures. Increased drag and weight generally result in decreased performance. In hull-type amphibians one disadvantage is entry to and exit from the aircraft. As the hull sits in the water, entry and exit usually occurs from the water or from a special dock built to accept these types of seaplanes. This can be a significant disadvantage as it may severely limit the aircraft's utility. Additionally, hull-type amphibians require buoyancy devices for lateral stability on the water as the center of buoyancy of the aircraft resides directly below the center of gravity; an unstable condition. These supplemental lateral stability devices are generally called sponsons and are typically mounted under each wing. The presence of the sponsons make docking and handling of a hull-type amphibian more difficult during water operations and increase drag during flight. Another disadvantage of a hull-type amphibian is that, unlike a float-plane where the operator can stand outside the aircraft on the floats, an operator of a hull-type amphibian has little area to stand to aid in maneuvering the aircraft while the aircraft is in the water. For example, it is common for a pilot of a float-plane to exit the aircraft and stand on one of the floats during water operations and use a paddle to maneuver the aircraft to a dock.
Sponsons of a float-plane provide supplemental buoyancy during water operations. During taxiing, landing and takeoff the sponsons and hull both experience what is called hydrodynamic hull drag. The trailing edge of the sponsons and hull act as a rear edge of a planing hull forming a hydrodynamic step. As is well known to one skilled in the relevant art, the use of a planing hull reduces the hydrodynamic hull drag at higher speeds because there is no trailing hull portion to generate negative pressure. A curved trailing hull is used for displacement type hulls to reduce negative pressure. But in a planing hull negative pressure can be substantially eliminated. Although pressure in a planing hull is primarily upward such that the hull rises higher and higher as speed increases, the majority of the hydrodynamic drag still occurs at the rear edge. The magnitude of this effect depends on whether the rear or aft portion of the hull is essentially flat. If the hull curves upward, a low pressure region is created thus increasing drag. If the hull curves downward, additional work is required to converge the water stream and there is excess churning of water which also creates drag.
Thus in a planing boat hull a hydrodynamic step is often formed by two surfaces meeting at approximately right angles. These surfaces are often the transom, which is almost vertical, and the planing surface bottom of the hull, which is generally horizontal at the rear of the boat. In an aircraft a vertical rear surface formed with a 90 degree angle is avoided because of increased aerodynamic resistance. Aerodynamic drag is negligible in a boat, however the drag caused by a vertical surface in an aircraft resulting in separated airflow can be substantial. Accordingly, a compromise must be reached between minimization of hydrodynamic drag during water operations and minimization of aerodynamic drag during flight operations.
Sponsons also provide lateral stability when the aircraft is at rest. However as an aircraft turns centrifugal forces roll the aircraft away from the direction of turn. The rolling motion, which is initially inhibited by the sponson, may cause the sponson, to become submerged if the speed of the turn and induced forces are significant. If the sponson becomes submerged drag significantly increases pulling the aircraft in the opposite direction thus rendering the turn impossible. As a result the aircraft must come to a stop, allowing the sponson to resurface so that the turn can continue or be reinitiated.
A challenge exists in the prior art for a configuration of a hull type amphibian possessing sponsons that can conduct a coordinated turn during water operations without significantly impacting the aircraft aerodynamic performance while airborne. A challenge also exists for sponson configured amphibious aircraft to turn at moderate speeds without submerging their sponsons. These and other challenges of the prior art are overcome by the present invention, which is hereafter described by way of example.